main: rename new_config arg of weston_wayland_backend_config_add_new_output
main: rename new_config arg of weston_wayland_backend_config_add_new_output

Rename new_config to config to follow legacy naming scheme.

Signed-off-by: Benoit Gschwind <6d0fcc95e39ab8ccb88bafebb2075e9d5c92b713@gnu-log.net>
Reviewed-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

krezovic/weston,giucam/weston,giucam/weston,Gnurou/weston,sir-murray/weston,Gnurou/weston,krezovic/weston,sir-murray/weston